Study of the magnetic equation of state as precursor of the area under the isothermal magnetocaloric potential

The isothermal evolution of magnetization under different magnetic fields, described by an equation of state, not only defines de entropy change ΔS(T) (magnetocaloric effect) but also the cooling power (area enclosed by the ΔS(T) curve). In fact, the area under the MT0(H) curve limited by the initial (Hi) and final (Hf) fields equals the area under the ΔS(T) curve above T>T0 for the same field range. This “sum rule” has been used to compare magnetocaloric curves for a number of materials. We based this study in the prediction that MT0(H) contains all the information to the establishment of the cooling power above T0 over the whole range of considered magnetic fields. To perform a check, we studied here some magnetic systems described by different equations of state including that around the transition region (power law). We show results of theoretical calculations in GdAl2-like ferromagnetic material with equation of state in the framework of Brillouin and Oguchi models. An intricate phenomenological equation of state for polycrystalline NdAl2 under hydrostatic pressure is also used to validate the sum-rule.